1. Field of the Invention
This invention relates to engine case structures of gas turbine engines, and specifically to compressor structures for use in conjunction with active control of clearances within an engine.
2. Description of the Prior Art
In a gas turbine engine of the type referred to above, working medium gases are pressurized in a compression section by a first series of rotor mounted blades and are flowed axially downstream to a combustion section. Fuel is combined with the pressurized gases and burned in the combustion section to add thermal energy to the flowing medium. In a turbine section downstream of the combustion section, the medium gases are flowed across a second series of rotor mounted blades. The second series of blades extracts energy from the flowing gases to drive the blades of the compression section.
In an axial flow engine the blades of the compression and turbine sections are arranged in rows which extend radially outwardly across the medium flowpath from an engine rotor. An essentially cylindrical case circumscribes the tips of the rotor blades. Vanes extend inwardly from the case between each pair of adjacent blade rows to direct the medium gases to a preferred angle of attack approaching each downstream blade row. Compression sections and turbine sections are formed by one of two construction techniques. In accordance with the first technique, rows of blades and vanes are assembled in alternating sequence within a cylindrical engine case. In the second technique, the cylindrical case is longitudinally split into a top and bottom segment. The blades are assembled onto the rotor and the vanes are assembled into the case segments. The case segments are then joined about the rotor to form the alternating blade and vane structure. The split case construction is illustrated, for example, in U.S. Pat. No. 2,848,156 to Oppenheimer entitled "Fixed Stator Vane Assemblies". The rows of stator vanes are assembled in the appropriate top or bottom segments such that when the top and bottom segments are joined, the rows of stator vanes will be alternatingly positioned between rows of rotor blades. Split case constructions afford ease of assembly and maintenance when compared to one piece, cylindrical cases.
The pressurized air of the compression section is utilized for providing aircraft services and for cooling applications within the turbine section of the engine itself. Additionally, substantial amounts of the pressurized air are bled from the compressor section to enhance starting characteristics and to decrease the sensitivity of the compressor to surge or stall phenomenon during operation. Manifolds collecting bleed air for these purposes are constructed about the compressor section of the engine. U.S. Pat. No. 3,597,106 to Anderson entitled "Combination Compressor Casing-Air Manifold Structure", for example, illustrates such a manifold which is contained entirely within the longitudinal split engine case.
In addition to the internal manifold of the type illustrated by Anderson, manifold structures are also known to be constructed externally of the engine case. When incorporated in longitudinally split case structures, however, external manifolds of the prior art are regional in nature and are not known to extend across the longitudinal split in the case. Regional manifolds are suitable for use where the temperature of the engine case is maintained at a near uniform level. Where deviations in case temperature are expected, the regional manifolds cause undesirable distortion of the case and attendant increases in clearance between the rotor and stator structures. If adequate initial clearance is not provided between the rotor and stator elements, destructive interference between such parts may also result.
Engine operating efficiency is largely dependent upon maintaining minimum clearances between the rotor and stator elements of the engine flowpath. For example, any clearance between the tips of the rotor blades and outer air seals of the engine case has a strongly negative effect on compression efficiency. Notwithstanding, the clearance must be sufficiently large to accommodate radial displacement of the rotor blade tips during acceleration of the engine as the temperature of the working medium gases increases. In response to increased temperature, the blades instantaneously expand in the spanwise direction outwardly toward the outer air seal. The outer air seal, however, responds with the compressor case from which it is supported at a much slower rate. Substantial initial clearance between the blade tips and the shroud is provided in the cold condition to prevent destructive impact of the blades on the shroud as the engine is accelerated and the flowpath temperatures increase. Unfortunately as thermally stable conditions are reached, the case and the outer air seal which is supported therefrom grow radially away from the blades leaving again a substantial clearance which approximates the initial clearance.
Recently developed active techniques for reducing the clearance between the rotor and stator elements at equilibrium conditions, are taught in U.S. Pat. Nos. 4,019,320 to Redinger et al entitled "External Gas Turbine Engine Cooling for Clearance Control" and 4,069,662 to Redinger et al entitled "Clearance Control for a Gas Turbine Engine". In such techniques, cooling air is flowed over the engine case to reduce the case diameter at equilibrium conditions such that the radial clearance between the rotor and stator elements is reduced to an acceptable minimum. The cooling techniques of the Redinger et al patents were developed primarily for one piece cylindrical cases and have yet to be successfully applied to the compression sections of engines employing longitudinally split compressor case structure.
The combination of active clearance control techniques into longitudinally split compressor structures presents several problems to which the concepts of the present invention are directed.